A conventional bistable liquid crystal display (LCD) has two stable liquid crystal (LC) configurations that can exist with no applied voltage. Switching between the two stable LC configurations is achieved via the application of a suitable voltage waveform, and the voltage waveform is not required to maintain either stable state but only to switch between the stable states. When combined with other optical components (e.g., polarizers), the two stable LC configurations have two optically distinct states. As used herein, “distinct optical states” means that the optical states correspond to light emissions that appear visually different to a viewer of the LCD. Because the voltage waveform is not required to maintain either stable state but only to switch between the stable states, no power is consumed in a stable LC configuration, and consequently bistable LCDs are attractive for their low power consumption. Bistable LCDs have been previously disclosed, for example, in U.S. Pat. No. 4,333,708 (Boyd et al., issued Jun. 8, 1982), U.S. Pat. No. 9,280,018 (Mottram et al., issue Mar. 8, 2016), U.S. Pat. No. 5,796,459 (Bryan-Brown et al., issued Aug. 18, 1998), and U.S. Pat. No. 6,903,790 (Kitson et al., issued Jun. 7, 2005).
Generally, a zenithal bistable alignment surface is an LC alignment surface that can adopt either a substantially vertically aligned state or a substantially horizontally aligned state with respect to the LC molecules at the alignment surface. LCDs described in U.S. Pat. No. 6,249,332 (Bryan-Brown et al., issued Jun. 19, 2001) have at least a first zenithally bistable alignment surface located on an opposite side of the LC layer from a monostable alignment surface (which can adopt only a single alignment state). In other embodiments, U.S. Pat. No. 6,249,332 also discloses an LCD that has two zenithally bistable alignment surfaces facing or opposing each other with the LC layer disposed between the two zenithally bistable alignment surfaces. An LCD with two zenithally bistable alignment surfaces may have four stable LC configurations that each can exist with no applied voltage, by virtue of the different combinations of the horizontal and vertical alignment states of the two zenithally bistable alignment surfaces. However, of the four stable LC configurations that may be achieved, two of the stable LC configurations are optically indistinguishable from each other owing to the arrangement of optical components described, i.e., two stable LC configurations appear identically to a viewer of the device, and thus are not optically distinct states. Consequently, in conventional configurations only three optically distinct LC states may be achieved using a configuration having two bistable alignment surfaces, because two of the stable states are optically identical rather than being optically distinct.